U.S. patent number 6,631,992 [Application Number 09/822,458] was granted by the patent office on 2003-10-14 for projector color correction to target white points.
This patent grant is currently assigned to Koninklijke Philips Electronics N.V.. Invention is credited to Matthew Scott Brennesholtz.
United States Patent |
6,631,992 |
Brennesholtz |
October 14, 2003 |
Projector color correction to target white points
Abstract
A system for adjusting the color of a light source comprises a
first polarizer to polarize the light source, a fixed retarder with
its optical axis at a first angle to the polarized light, an
electronically variable retarder with its optical axis at a second
angle to the polarized light and a second polarizer to polarize the
resultant light. This light source may be used where light of
adjustable color is needed, for example in a theatrical spotlight
or a liquid crystal projection system.
Inventors: |
Brennesholtz; Matthew Scott
(Pleasantville, NY) |
Assignee: |
Koninklijke Philips Electronics
N.V. (Eindhoven, NL)
|
Family
ID: |
25236092 |
Appl.
No.: |
09/822,458 |
Filed: |
March 30, 2001 |
Current U.S.
Class: |
353/20; 349/9;
353/8; 353/31; 353/37; 353/34; 348/E9.027; 359/491.01;
359/489.07 |
Current CPC
Class: |
H04N
9/3167 (20130101); H04N 9/3114 (20130101); H04N
9/3182 (20130101) |
Current International
Class: |
H04N
9/31 (20060101); G03B 021/00 (); G03B 021/14 ();
G03B 021/26 (); G02F 001/133 (); G02B 005/30 () |
Field of
Search: |
;353/8,20,31,33,34,37
;349/8,9 ;359/497,501,502 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
|
|
|
|
|
|
|
0390511 |
|
Oct 1990 |
|
EP |
|
0909974 |
|
Apr 1999 |
|
EP |
|
7294869 |
|
Nov 1995 |
|
JP |
|
Primary Examiner: Adams; Russell
Assistant Examiner: Cruz; Magda
Claims
I claim:
1. A system for adjusting a color of a light source along an
illumination path, comprising: first integrator plate positioned
along the illumination path; a first polarizer positioned along the
illumination path downstream from said first integrator plate, said
first polarizer to polarize the light source a fixed retarder
positioned along the illumination path downstream from said first
integrator plate, said fixed retarder having a first optical axis
at a first angle to the illumination path an electronically
variable retarder positioned along the illumination path downstream
from said first integrator plate, said electronically variable
retarder having a second optical axis at a second angle to the
illumination path; and a second polarizer positioned along the
illumination path downstream from said first integrator plate, said
second polarizer to polarize the polarized light; and a second
integrator plate positioned along the illumination path downstream
from said first integrator plate.
2. The system of claim 1, wherein the polarized light is partially
polarized.
3. The system of claim 1, wherein said first polarizer includes a
polarization conversion system.
4. The system of claim 1, wherein said fixed retarder is a quarter
wave retarder.
5. The system of claim 1, wherein said variable retarder varies in
a range of 700-110 nm.
6. The system of claim 1, further comprising: a spatial light
modulator, wherein the light source is used to illuminate said
spatial light modulator.
7. The system of claim 6, wherein said spatial light modulator is a
liquid crystal device.
Description
INDUSTRIAL FIELD OF APPLICATION
The invention is generally concerned with adjusting the color of a
light to a desired, or "target," white. This is useful, for
example, in spotlights for theatrical or film work. The invention
is also concerned with adjusting the white of a liquid crystal
display ("LCD") to a target white. The invention is adaptable to
any LCD system, whether projection or direct view.
BACKGROUND OF THE INVENTION
Many different colors are specified as "white" in video and monitor
systems. For example, 5400.degree. K is specified as white for
cinema work, but 6500.degree. K is specified as white for video
work. White colors form 3400.degree. K to 9300.degree. K are used
for other applications. For an end user to accurately judge color
of the finished product on a monitor, it is necessary to adjust the
monitor white point to the white point specified for the particular
application.
In CRT monitors, this can be accomplished by adjusting the relative
gains of the red, green and blue video amplifiers so that when all
three video signals (R, G an B) are equal, the desired target white
is displayed. Changing the white point involves changing the
relative gain of the red, green and blue amplifiers.
This type of system has several disadvantages when it is used on a
LCD display. If the gain is done in the analog domain, it affects
not only the white point, but, because of the non-linear nature of
LCDs, all other colors as well. This then requires modified Gamma
correction curves for each target white point. If the gain
adjustment is done in the digital domain, it requires that some of
the dynamic range be ignored and not used. Since 8 bits is only
marginally enough bits for any serious color work, ignoring part of
the dynamic range can lead to serious color artifacts. Even if the
basic resolution is 10 or 12 bits, color artifacts can result from
interpolating the video signal to a smaller dynamic range.
Therefore, it is desirable to be able to adjust the color of a LCD
display (projection or direct view) without changing the input
video signal in either the analog or digital domains. The
adjustment should be optical and controllable while the projector
is operating.
EP 909 974 covers the use of one or two retarders with a reflective
LCD cell to make the system achromatic and improve the contrast.
The retarder(s) are with the LCD between the polarization-sensitive
beam splitter ("PBS") and the mirror. The system of the invention
puts the retarders in the illumination path so they have no effect
on contrast. The system is not designed to minimize chromatic
effects. Rather, the system of the invention is designed to add
exactly the right amount of chromatic effects to allow color
correction to target whites.
U.S. Pat. No. 5,875,008 describes the use of three sets of
compensators, one with each LCD to adjust color point, efficiency
and contrast of the individual LCDs. While one effect of this is to
change the white point, that is not the goal; and the approach is
different from the approach of the invention. Again, the
compensators are not in the illumination path.
EP 390,511 describes the use of three sets of one or two
compensators, one with each LCD to adjust color point, efficiency
and contrast of the individual LCDs. While one effect of this is to
change the white point, that is not the goal and the approach is
different from the approach of the invention. Again, the
compensators are not in the illumination path.
Kokai 07-294,869 describes a system that adjusts the polarizers of
at least two LCDs to adjust the efficiency of the cell. This then
adjusts the color balance and the white point. The system of the
invention adjusts retarders, not polarizers, and one common
retarder is used for all three colors. Adjusting polarizers
involves a mechanical system. Adjusting retarders can be done
electronically.
Application PHN17544 EP-P describes a method for adjustment of a
desired white color of at least a portion of an image, by a system
provided with a light source and an electro-optical modulator,
wherein light from the light source is modulated by the
electro-optical modulator into the image, wherein the system
comprises with the electro-optical modulator cooperating, a light
adjustment device, wherein the electro-optical modulator is driven
for at least a portion to a desired maximum, the portion being
imaged on the portion of the image, after which the color of said
portion of the image is adjusted to the desired white color by the
light adjustment device.
A device includes a cooperating light adjustment device for each of
the respective LCD panels. Therefore, a two LCD panel projector
comprises two light adjustment panels, and a three LCD panel
projector comprises three light adjustment panels. Also an
embodiment is described in which a stack of light adjustment
devices is used, but still the number of light adjustment devices
corresponds to the number of electro-optical modulators.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide a system and a
method for adjusting the color of a light to a target white.
It is an object of the present invention to provide a system and a
method for adjusting the target white of a LCD system.
It is another object of the present invention to provide such a
system that is adapted to either a projection or direct view LCD
system.
It is an object of the present invention to provide a simple and
reliable electronic system for adjusting the target white of a LCD
system.
These objects and others that will become apparent from the
following specification are achieved by a system for adjusting the
color of a light source which comprises the following components:
a) a first polarizer to polarize the light source, b) a fixed
retarder with its optical axis at a first angle to the polarized
light, c) an electronically variable retarder, with its optical
axis at a second angle to the polarized light, and d) a second
polarizer to polarize the resultant light.
It is preferred that the system consists essentially of components
a) to d). It is also preferred that the components be present in
the order a) to d). Alternatively, the components may be in the
order a), c), b) and d). Preferably, the polarized light is
partially polarized. The first polarizer to polarize the light
source may comprise a polarization conversion system. Preferably,
the fixed retarder is a quarter wave retarder. The variable
retarder can vary in the range of 700-1100 nm. Preferably, the
light source is used to illuminate a display with one or more
spatial light modulators. It is preferred that the spatial light
modulator(s) be liquid crystal device(s).
The invention comprises a system for color correction. One version
of the system uses the partially polarized light produced by an
Integrator/Polarization Conversion System (PCS) combination and an
ECB cell and one or more fixed compensator foils. By adjusting the
voltages on the ECB cell(s), it is possible to adjust the relative
amounts of red, green and blue light reaching the LCDs and
therefore adjust the color temperature of the projector. Since all
adjustments are done electronically, not mechanically, the system
is simple and reliable.
The color correction scheme requires that the basic colorimetry of
the display system be correct for one white point. This approach
then allows the white point color to be adjusted over sufficient
range to allow correction to other white points. The system has
been tested with the optics from the color drum projector, U.S.
patent application Ser. No. 09/127,003 but would be adaptable to
any LCD system, whether projection or direct view.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a simplified general diagram of the color correction
system of the invention with a variable retarder.
FIG. 2 is a diagram of a color correction system used to generate
the measured data in FIGS. 3 and 4.
FIG. 3 is a graph of the transmission of the quarter wave and
variable retarders between the polarization-sensitive beam splitter
and the polarization conversion system.
FIG. 4 is a graph showing the adjustment gamut of the color drum
projector with the quarter wave and variable retarders.
PREFERRED EMBODIMENTS OF THE INVENTION
FIG. 1 is a simplified general diagram of the color correction
system of the invention with a variable retarder. The light with
the initial color from lamp 30 is reflected by reflector 31 to the
right, where it passes through a first polarizer 32, where it is
partially polarized with horizontal polarization. The partially
polarized light then passes through fixed retarder 33 and then
through a variable retarder 34, and the retardance is reduced as
voltage is applied by the variable retarder controller 35. After it
passes through the second polarizer 36, it may be used as a
spotlight or be passed to one or more spatial light modulators 37,
which may be either reflective or transmissive, e.g. a liquid
crystal device.
As shown in FIG. 2, the white point color adjustment involves a
single ECB cell (or other electronically variable retarder) and a
fixed retarder. Due to the physical size of the available variable
retarder from Meadow Lark, it was impossible to put the system in
the complete drum projector. Components from the drum projector
were installed on an optical rail for testing, as shown in FIG.
2.
In the description of this system, all azimuthal angles about the
optical axis are positive when they are clockwise as seen from the
lamp 10. The light from lamp 10 is reflected by reflector 11 to the
right, where it passes through a first integrator plate 12 and then
to second integrator plate 13. The output of the polarization
conversion system 14 unit is partially polarized with horizontal
polarization.
The fast axis of the quarter wave plate 15 was set at -10.degree..
The variable retarder 16 used in this test had a retardance of 1100
nm when no voltage was applied, and the retardance was reduced as
voltage was applied. The output of square wave generator 17 was
input to variable retarder 16 in the setup for which the results
are given in FIGS. 3 and 4, and the axis of the variable retarder
16 was set at +45.degree.. Other combinations of retardance value
(i.e., other than quarter wave), angle of the retarder 15 and angle
of the variable retarder 16 give similar results, with different
drive voltages on the variable retarder 16. For these results, a
square wave generator 17 output a 360 Hz square wave with rms
values in the range of 3.2V to 4.3V. The use of 360 Hz was a
limitation of the variable retarder drive system. More commonly, a
higher frequency such as 1 kHz would be used. There is no need to
have the variable retarder frequency tied to a multiple of the
video field rate.
After it passes through polarization conversion system 14, the
light is approximately 70% horizontal polarized. This is equivalent
to 90.degree. azimuthal angle for the e-field, and it is
p-polarized at the polarization-sensitive beam splitter (PBS) 18.
The remaining 30% of the light is vertically polarized, (0.degree.
or s- at the polarization-sensitive beam splitter (PBS) 18). In
normal operation without the present invention, this s-polarized
light reflects at the polarization-sensitive beam splitter (PBS) 18
and is lost to the system. The light from the PBS 18 enters a 3"
integrating sphere 19, where its wavelength and color are sampled
by fiber optic probe 20 and measured by spectrometer 21, sampling
the light from 3" integrating sphere 19. The intensity of the light
is measured by photometer 22, which works in conjunction with a
photometric detector 23.
FIG. 3 shows the transmission of the polarization conversion
system/quarter wave retarder/variable
retarder/polarization-sensitive beam splitter system as a function
of voltage and wavelength. This measured data was normalized to a
100% level equal to the light transmitted by the polarization
conversion system/polarization-sensitive beam splitter system with
no fixed or variable retarder in the path. The data were corrected
for the transmission in unpolarized light of the quarter wave plate
and the variable retarder. This transmission was 82.2% (photometric
with the UHP lamp) in the test setup. With proper optical coupling
and anti-reflective coating, this could be expected to be >98%
in a production system. Note that the peak transmission it actually
>100%. (106.9%) It is not known if this value is an artifact of
the measurement or if it is real. Due to the presence of the
s-polarized light that normally would be discarded, a transmission
of >100% is not physically impossible, although it is probably
not likely either.
Note that at every voltage, there is a wavelength where the peak
transmission is equal to the maximum possible. This is a very
desirable feature in a color correction system because it means one
is not discarding light unnecessarily. As the voltage varies, the
maximum transmission varies from the red region at low voltages,
through the green region at medium voltages to the blue region at
higher voltages.
When these transmissions are convolved with the drum projector
spectral throughput efficiencies, a range of colors is produced.
This range of colors is shown in FIG. 3. This graph was produced
assuming the basic white color was corrected to a color slightly
more greenish than 6550.degree. K prior to the white adjustment.
Because there is only one degree of freedom, it is only possible to
adjust the color along one line. This line corresponds
approximately with the black body line. Since all target white
points are on or near the black body link, this is not a major
problem.
While the adjustment gamut line in FIG. 4 does not exactly match
the black body line, it is close enough to the black body line and
parallel enough to it to produce the desired range of white
colors.
The fixed/variable retarder system described above requires
partially polarized light for correct operation. If fully polarized
light is used, the slope of the color adjustment gamut line becomes
too steep compared to the slope of the black body curve. In a
system with a polarization conversion system, this is not a problem
because the light after the polarization conversion system is
partially polarized in all present systems. In a system where the
light is fully polarized, it would be necessary to design the
fixed/variable retarder system with different retardance values
and/or angles between the retarders and the polarized light in
order to produce the desired type of correction.
In a system where the light is unpolarized (i.e., a system without
a polarization conversion system), it is necessary to partially
polarize the light for the fixed/variable retarder color correction
system to work correctly. One way to accomplish this, for example,
is to use one or more Brewster plates in the illumination path
prior to the color correction system. These could be inserted, for
example, between the two integrator plates in a LCD projection
system in order to avoid lengthening the optical path.
By adjusting the retardance and angle of the fixed retarder and the
angle of the variable retarder, it is possible to change their
slope of the adjustment gamut shown in FIG. 4 so it becomes more
parallel to the black body curve, if desired. This would be done
during the design phase and would not be user adjustable. The user
would only be able to adjust the retardance of the variable
retarder by adjusting the voltage across the retarder.
The foregoing specification and drawings have thus described and
illustrated a system and a method for adjusting the target white of
a lighting system, particularly the target white of an LCD system,
which fulfill all the objects and advantages sought therefor. Many
changes, modifications, variations and other uses and applications
of the subject invention will, however, become apparent to those
skilled in the art after considering this specification which
discloses the preferred embodiments thereof. All such changes,
modifications, variations and other uses and applications which do
not depart from the spirit and scope of the invention are deemed to
be covered by the invention, which is to be limited only to the
claims which follow.
* * * * *